The present invention relates to a coordinate-measuring machine having a table to receive a workpiece to be measured and a device which expands the usable measurement volume of the coordinate-measuring machine.
In coordinate-measuring machines, the area of a granite plate forms the machine bed and also serves as the table for receiving a workpiece to be measured, and this granite area is substantially larger than the usable measurement range of the machine in the horizontal plane, i.e., in the direction of the X and Y axes. In a portal-type machine, this is due, among other things, to the fact that a fraction of the surface area of this granite plate must also serve directly as a guide surface, for the displaceable portal; this fraction of the surface area must therefore remain free of obstruction.
In the direction of travel of the portal, the guide surface must always be longer than the travel path, namely, by at least the base length of the guided base regions of the portal columns. For reasons of stability, this base length is greater, the greater the height of the portal. The guided base regions of the portal columns require spaced travel paths along relatively wide lateral margins of the granite plate; these lateral margins of the granite plate must be outside the measurement volume of the machine and cannot be used either for workpiece-clamping or for mounting a probe-exchange device. These considerations are factors making the machine more expensive as well as unnecessarily large and heavy.
It has already been proposed to expand the measurement range of a coordinate-measuring machine of the portal type, in the direction perpendicular to the direction of portal travel, by extending the bridge or cross-arm so as to project outwardly on both lateral sides beyond the feet of the portal columns, thereby extending the Y-axis guide, for a cross-slide traveling thereon. Such a machine is described, for example in West German Patent No. 3,024,455.
This measure of laterally outward projection cannot be readily used for increasing the measurement range in the direction of travel of the portal since a lengthening of the guide surface means nothing else but a lengthening of the granite plate. Even if those parts of the table which are outside the guide paths were to be removed, the granite plate would nevertheless have to be produced from a larger block since the guide paths must consist of one piece. Furthermore, a lengthening of the guide paths has the result that the scale for measuring the position of the portal in the direction of travel becomes correspondingly long. And the creation of long scales results in disproportionately high expense once a given maximum length is exceeded.
West German published Application OS 3,729,433 describes a coordinate-measuring machine of the portal type wherein a recess is formed in the granite plate so that workpieces can be measured even if their height exceeds the height of the measurement volume. In use, the upper part of the workpiece which extends out the recess is first measured, and the workpiece is then lifted out by means of a crane, inverted, inserted again into the recess, and then measured with respect to another side. This is a very cumbersome and time-consuming method. The measurement range is not increased by these measures.
West German Patent No. 1,798,191 describes a coordinate-measuring machine which has a table which can be adjusted vertically in several steps. In this machine, the vertical distance between the probe head and the lower position of the table top is relatively large as compared with the measurement range in the direction of the Z axis, i.e., with respect to the range of displacement of the vertical spindle of the machine. Thus, even relatively high workpieces can be measured and for flat workpieces the table is simply displaced upward to a given minimum height. Again, however, the measurement range is not increased by this technique.
West German published Application OS 3,527,384 discloses a one-dimensional length-measuring machine, the measurement slide of which is guided indirectly in a second slide which, in its turn, is guided for displacement in the same direction on a stationary machine table. The scale which measures the position of the probe pin in the direction of displacement is scanned by two pick-up heads, one of the heads being lockable in different positions.
If one wanted to apply these measures to multiple-coordinate measuring machines, then twice the number of highly accurate guides would accordingly have to be provided. In the case of portal-type machines, the portal, which is in any event already very heavy, would then have to be guided on a second displaceable slide. This would be cumbersome and would entail disproportionately high expense.